throttle

A throttle is the mechanism by which the flow of a fluid is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases (i.e., by the use of a throttle), but usually decreased. The term throttle has come to refer, informally and incorrectly, to any mechanism by which the power or speed of an engine is regulated. What is often termed a throttle is more correctly called a thrust lever.

Internal combustion engines

In a petrol (gasoline) internal combustion engine, the throttle is a valve that directly regulates the amount of air entering the engine, indirectly controlling the fuel burned on each cycle due to the fuel-injector or carburetor maintaining a relatively constant fuel/air ratio. In a motor vehicle the control used by the driver to regulate power is sometimes called the throttle pedal or accelerator.

The throttle is typically a butterfly valve. In a fuel-injected engine, the throttle valve is housed in the throttle body. In a carbureted engine, it is found in the carburetor.

When a throttle is wide open, the intake manifold is usually at ambient atmospheric pressure. When the throttle is partially closed, a manifold vacuum develops as the intake drops below ambient pressure.

Usually the throttle valve is mechanically linked with the throttle pedal or lever. In vehicles with electronic throttle control, the throttle valve is electronically controlled, which allows the ECU greater possibilities in reducing air emissions.

In a reciprocating-engine aircraft, the throttle control is usually a hand-operated lever or knob. It controls the engine power, which may or may not reflect in a change of RPM, depending on the propeller installation (fixed-pitch or constant speed).^[1]

Diesel engines do not need to control air volumes. Thus they lack a butterfly valve in the intake tract, and do not have a throttle (although recent developments in Exhaust Gas Recirculation have introduced throttle-style designs^[2]). They instead regulate engine power by directly controlling the quantity of fuel injected into the cylinder just before top dead centre (TDC) of the compression stroke.

Throttle body

The components of a typical throttle body

In fuel injected engines, the throttle body is the part of the air intake system that controls the amount of air flowing into the engine, in response to driver accelerator pedal input in the main. The throttle body is usually located between the air filter box and the intake manifold, and it is usually attached to, or near, the mass airflow sensor.

The largest piece inside the throttle body is the throttle plate, which is a butterfly valve that regulates the airflow.

On many cars, the accelerator pedal motion is communicated via the throttle cable, to activate the throttle linkages, which move the throttle plate. In cars with electronic throttle control (also known as "drive-by-wire"), an electric motor controls the throttle linkages and the accelerator pedal connects not to the throttle body, but to a sensor, which sends the pedal position to the Engine Control Unit (ECU). The ECU determines the throttle opening based on accelerator pedal position and inputs from other engine sensors.

Throttle body showing throttle position sensor. The throttle cable attaches to the curved, black portion on the left. The copper-coloured coil visible next to this returns the throttle to its idle position when the pedal is released.

When the driver presses on the accelerator pedal, the throttle plate rotates within the throttle body, opening the throttle passage to allow more air into the intake manifold. Usually an airflow sensor measures this change and communicates with the ECU. The ECU then increases the amount of fuel being sent to the fuel injectors in order to obtain the desired air-fuel ratio. Often a throttle position sensor (TPS) is connected to the shaft of the throttle plate to provide the ECU with information on whether the throttle is in the idle position, wide-open throttle (WOT) position, or somewhere in between these extremes.

Throttle bodies may also contain valves and adjustments to control the minimum airflow during idle. Even in those units that are not "drive-by-wire" there will often be a small electric motor driven valve, the Idle Air Control Valve (IACV), that the ECU uses to control the amount of air that can bypass the main throttle opening.

Image of BMW S65 from the e92 BMW M3 showing eight individual throttle bodies

Many cars have a single throttle body, however more than one may be used, chained together by linkages, to improve throttle response. At the extreme, high performance cars, such as the BMW M1, and high performance motorcycles, like the Suzuki Hayabusa, have a separate throttle body for each cylinder. These are often referred to as "individual throttle bodies", or ITBs.

A throttle body is somewhat analogous to the carburetor in a non-injected engine. Carburetors combine the functionality of the throttle body and the fuel injectors into one, that is, to modulate the amount of air flow, and to combine air and fuel together. Cars with throttle body injection (called TBI by General Motors and CFI by Ford) locate the fuel injectors in the throttle body, thereby allowing an older engine to be converted from carburetor to fuel injection without significantly altering the engine design.

Environmental aspects

Regulation of the throttle is also a mechanism for controlling engine exhaust emissions. In many modern internal combustion engines an electronic throttle is used, eliminating the older accelerator cable.^[3]

Throttle application via the accelerator pedal also results in increased sound emission from the engine. At lower operating speeds this component of vehicle noise is prominent, contrasted with higher operating speeds, for which aerodynamic and tire noise are more significant.^[4]

Other engines

Most engines have some kind of throttle control, though the particular way that power is regulated is often different.

Liquid rockets are throttled by controlling the pumps which send liquid fuel and oxidizer to the combustion chamber. Solid rockets are not controlable once the solid fuel is ignited. Hybrid Rockets, like the one used in Space Ship One, use solid fuel and liquid oxidizer, thus throttling is possible through the oxygen pumps, like in a full liquid rocket motor.

In a jet engine, engine output is also directly controlled by changing the amount of fuel flowing into the combustion chamber, usually with an autothrottle. In some instances, a "throttle" is known as a "thrust lever" (as in most Airbus and Boeing aircraft). This is chiefly due to the fact that a standard "throttle" is associated with internal combustion engines.^[5]

References

External links

Look up throttle in Wiktionary, the free dictionary.